Evaluating a vortex flow-meter signal

ABSTRACT

A signal processor, for use with a zero crossing module of a vortex flow meter, includes a peak amplitude detector, a comparator, and a filter module. The filter module is enabled when the comparator determines that the amplitude is less than a low flow rate threshold. The filter module filters a vortex signal and the filtered signal is provided to a frequency estimator that uses a zero crossing algorithm. The signal processor may increase the signal-to-noise ratio at low flow rates for which the frequency estimator may not otherwise be able to accurately estimate the flow rate. Low flow rates may be measured by determining that there is a low flow rate using amplitude detection of a vortex signal, filtering the vortex signal based on the amplitude detection, and using a zero crossing algorithm on the filtered,vortex signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. ProvisionalApplication No. 60/300,421, filed Jun. 26, 2001, and titled “Filteringand Analysis System for Evaluating a Vortex Flow meter System,” which isincorporated by reference.

TECHNICAL FIELD

[0002] Certain implementations relate generally to evaluating a signal,including analyzing and filtering a signal, and more particularly toanalyzing and filtering a vortex flow meter signal.

BACKGROUND

[0003] Flow meters may measure the rate of flow of a fluid in a pipe orother pathway. The fluid may be, for example, a gas or a liquid, and maybe compressible or incompressible. One type of flow meter is a vortexflow meter which is based on the principle of vortex shedding. Vortexshedding refers to a natural process in which a fluid passing a bluffbody causes a boundary layer of slowly moving fluid to be formed alongthe surface of the bluff body. A low pressure area is created behind thebluff body and causes the boundary layer to roll up, which generatesvortices in succession on opposite sides of the bluff body. The vorticesinduce pressure variations that may be sensed by a pressure sensor. Thevortex-shedding pressure variations have a frequency that is related tothe flow rate. Accordingly, by measuring the frequency of the pressurevariations, the flow rate may be determined.

SUMMARY

[0004] One technique for measuring the frequency of vortex-sheddingpressure variations includes converting the pressure values into anelectric signal and determining the time between zero crossings of theelectric signal. The term “vortex signal” is used to refer to theelectric signal or, more generally, and depending on the context, torefer to the pressure variations or some other signal derived from thepressure variations. Inverting the time between zero crossings yieldsthe frequency of the vortex signal. However, zero crossings of thevortex signal may be difficult to determine, particularly for low flowrates that may produce pressure variations of a lower magnitude thanpressure variations produced for high flow rates. Determining zerocrossings may also be difficult in the presence of noise. Noise may bepresent from, for example, turbulence and “plant noise” such as, forexample, pump vibrations and pipe-line vibrations. A lower magnitude forthe vortex signal and/or the presence of noise may result in a lowersignal-to-noise ratio (“SNR”) for the vortex signal at low flow rates.Low SNRs may make it difficult to lock on to the vortex signal and/or totrack the vortex signal using a zero crossing technique.

[0005] Low flow rates, and the associated low SNRs, may occur, forexample, in at least three scenarios. The first scenario may occur whena flow meter has not acquired a lock on the vortex signal because theflow rate is low. For example, at start-up, a flow meter may not knowthe frequency of the vortex signal and, accordingly, may not filter outany noise due to the possibility that the vortex signal could also befiltered out. The second scenario may occur when a flow meter hasacquired a lock on the vortex signal and is able to filter out noisethat is not too close to the vortex signal frequency, but still cannottrack the vortex signal to a lower flow rate because of the low SNR. Thethird scenario may occur when a flow meter has acquired a lock and istracking the vortex signal at a low flow rate, but intermittent noisecauses the flow meter to lose the lock and/or to track the noise.

[0006] Each of these scenarios, and others, can be addressed byproviding the flow meter with additional functionality that determinesthat a flow rate is low and filters some of the noise out of the vortexsignal. The flow rate may be determined to be low, for example, bydetermining that the amplitude of the vortex signal is low. The flowrate can be determined from the amplitude because the flow rate isdirectly related to the amplitude. The filtering may be done using, forexample, a band-pass filter (“BPF”) having one or more pass bands.Filtering the vortex signal to remove noise may increase the SNR of thevortex signal and focus the flow meter on a smaller range of (low) flowvalues, which may help the flow meter to lock on and/or track the vortexsignal.

[0007] Using amplitude detection to determine that the flow rate is lowmay also be more robust to noise than determining zero crossings at lowflow rates. Thus, the amplitude detection may be expected to determinethat the flow rate is low even when the zero crossing detection cannot.The amplitude detection may also include filtering. For example, anamplitude detector may detect peaks in the vortex signal and these peaksmay be filtered to reduce the effect of noise on the peak measurements.In this way, the determination that the flow rate is low may be lesslikely to be changed inadvertently and to interrupt the correspondingfiltering.

[0008] According to a general aspect, a signal processor for use with azero crossing module of a vortex flow meter includes a peak detector, acomparator, and a filter module. The peak detector produces an amplitudeestimate. The comparator is coupled to the peak detector and receivesthe amplitude estimate and a threshold amplitude. The comparatorcompares the amplitude estimate and the threshold amplitude to produce acomparison result. The filter module is coupled to the comparator andreceives the comparison result and a signal. The filter module isoperable to selectively filter the signal based on the comparison resultand to provide the selectively filtered signal to a zero crossingmodule.

[0009] A peak filter may be disposed between the peak detector and thecomparator. The peak filter may filter the amplitude estimates producedby the peak detector to produce a filtered amplitude estimate. Thecomparison result may indicate whether the amplitude estimate is lessthan the threshold amplitude, and the filter module may filter thesignal using a first pass band if the comparison result indicates thatthe amplitude estimate is less than the threshold amplitude. The filtermodule may filter the signal using a second pass band if the first passband is not used, or regardless of whether the first pass band is used.The second pass band may include a variable pass band that depends on anestimated vortex frequency of the signal. The filter module may includea first filter and a second filter. The first filter may be coupled tothe second filter, and may selectively filter the signal using the firstpass band. The second filter may selectively filter the signal using thesecond pass band.

[0010] According to another general aspect, a vortex flow meter includesa peak detector, a comparator, a filter module, and a frequencyestimation module. The peak detector is operable to produce an amplitudeestimate. The comparator is coupled to the peak detector, and receivesand compares the amplitude estimate and a threshold amplitude to producea comparison result. The filter module is coupled to the comparator andincludes at least one filter. The filter module receives the comparisonresult and a signal, selectively filters the signal based on thecomparison result, and provides the selectively filtered signal as anoutput. The frequency estimation module is coupled to the filter moduleand includes a zero-crossing detector and a frequency estimator. Thefrequency estimation module-receives the selectively-filtered signal,detects zero crossings in the selectively-filtered signal, and estimatesa vortex frequency of the selectively-filtered signal based on thedetected zero crossings.

[0011] A peak filter may be disposed between the peak detector and thecomparator. The peak filter may filter the amplitude estimates producedby the peak detector to produce a filtered amplitude estimate. Thecomparison result may indicate whether the amplitude estimate is lessthan the threshold amplitude, and the filter module may filter thesignal using a first pass band if the comparison result indicates thatthe amplitude estimate is less than the threshold amplitude. The filtermodule may filter the signal using a second pass band if the first passband is not used, or regardless of whether the first pass band is used.

[0012] According to another general aspect, processing a vortex signalin a vortex flow meter includes comparing an amplitude of a vortexsignal to a threshold amplitude, producing an indication of whether theamplitude of the vortex signal is less than the threshold amplitude,filtering the vortex signal using a first pass band only if theamplitude of the vortex signal is less than the threshold amplitude,filtering the vortex signal using a second pass band if the first passband is not used, detecting zero crossings of the filtered vortexsignal, and estimating a vortex frequency based on the detected zerocrossings.

[0013] The threshold amplitude may reflect a low flow rate, such thatthe vortex signal is filtered using the first pass band only if the flowrate is low. The first pass band need not vary with the amplitude of thevortex signal. The threshold amplitude may be adjusted by a hysteresisvalue. Detecting the amplitude of the vortex signal may includedetecting peaks of the vortex signal and filtering the detected peaks toreduce high-frequency components. The amplitude of the vortex signal mayinclude a detected amplitude.

[0014] According to another general aspect, determining a flow rate of afluid includes determining that a flow has a low flow rate by detectingan amplitude of a vortex signal; filtering the vortex signal to reduce ahigh frequency component based on the determination that the flow has alow flow rate; and determining a flow rate of the flow using a zerocrossing algorithm on the filtered vortex signal.

[0015] Detecting an amplitude may include detecting peaks in theamplitude of the vortex signal and filtering the detected peaks in theamplitude of the vortex signal to remove a high-frequency component.

[0016] The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features will beapparent from the description and the drawings, and from the claims.

DESCRIPTION OF DRAWINGS

[0017]FIG. 1 is a block diagram of an example of a vortex flow meterhaving a pre-filter and an amplitude detector.

[0018]FIG. 2 is a block diagram of an example of the amplitude detectorof FIG. 1.

[0019]FIG. 3 is a block diagram of a primarily digital implementation ofa vortex flow meter.

[0020]FIG. 4 is a flow chart of a process for propagating a signalthrough a vortex flow meter having a pre-filter and an amplitudedetector.

[0021]FIG. 5 is a flow chart of a process for determining if a vortexsignal has an amplitude that is less than a threshold amplitude.

[0022]FIG. 6 is a set of graphs providing simulation data for a start-upat a low flow rate of a vortex flow meter having a pre-filter.

[0023]FIG. 7 is a set of graphs providing data from a first simulationas a flow rate is varied and decreased to a low flow rate.

[0024]FIG. 8 is a set of graphs providing data from a second simulationas a flow rate is varied and decreased to a low flow rate.

[0025]FIG. 9 is a set of graphs providing simulation data at a low flowrate as noise spurs are introduced.

[0026] FIG 10 is a block diagram of an example of a vortex flow meterhaving a combined pre-filter and band-pass filter.

[0027]FIG. 11 is an axial cross-sectional view of an example of a bluffbody in a pipe.

DETAILED DESCRIPTION

[0028] Architecture

[0029] Referring to FIG. 1, a system 100 may be used as a vortex flowmeter to measure flow rate using the vortex shedding principle. Thesystem 100 includes a pressure sensor 110 that senses the pressure of afluid and produces as an output a signal representing the pressure. Theoutput of the pressure sensor 110 is provided to a pre-amplifier 120that amplifies the signal. The output of the pre-amplifier 120 isprovided to both a pre-filter 130 and an amplitude detector 140. If thepre-filter 130 is enabled, the pre-filter 130 filters the input signalto remove noise. If the pre-filter 130 is not enabled, the pre-filter130 passes the signal through to the output without filtering. Theamplitude detector 140 detects the amplitude of the input signal andenables the pre-filter 130 if the detected amplitude is below athreshold value. The output of the amplitude detector 140 thus providesan ON/OFF signal to the pre-filter 130. The output of the pre-filter 130is provided to a band-pass filter (“BPF”) 150, the output of which isprovided to an analog-to-digital (“A/D”) converter (“ADC”) 160. The BPF150 filters the input signal to remove noise and the ADC 160 convertsits input signal from analog to digital. The output of the ADC 160 isprovided to a ZCA module 170 that executes a zero crossing algorithm(“ZCA”) and estimates a frequency of the vortex signal. The ZCA module170 provides one or more filter settings to the BPF 150. The output ofthe ZCA module 170 is provided to a smoothing filter 180. The smoothingfilter 180 produces a smoothed frequency estimate and may be coupled toa flow estimator (not shown) that estimates the flow rate of the fluidbased on the frequency of the vortex signal.

[0030] Referring to FIG. 2, a system 200 may be used as the amplitudedetector 140 in the system 100. The system 200 includes an absolutevalue converter 210. The absolute value converter 210 determines theabsolute value of an input signal and provides that absolute value as anoutput to a peak detector 220. The peak detector 220 detects orestimates the peaks in the input signal and provides those peak values,or some representation of them as an output to a low pass filter (“LPF”)230. The LPF 230 filters the input signal and provides the filteredsignal as an output to a comparator 240. The comparator 240 alsoreceives a threshold amplitude signal A-bar. The comparator 240 comparesthe input filtered signal and A-bar and produces an output based on thecomparison. The output of the comparator 240 may be used to enable thepre-filter 130 if the filtered input is less than A-bar and to disablethe pre-filter 130 otherwise.

[0031] Referring to FIG. 3, a system 300 may be used as a primarilydigital implementation of a vortex flow meter. The system 300 includes apressure sensor 310 that corresponds to the pressure sensor 110 andprovides output to a pre-amplifier 320 that corresponds to thepre-amplifier 120. The output of the pre-amplifier 120 is provided to anADC 360 that corresponds to the ADC 160. The digital output of the ADC360 is provided to a pre-filter 330 and an amplitude detector 340 thatcorrespond, respectively, to the pre-filter 130 and the amplitudedetector 140. The output of the pre-filter 330 is provided to a BPF 350that corresponds to the BPF 150 and provides an output to a ZCA module370 that corresponds to the ZCA module 170. One output of the ZCA module570 is provided to a smoothing filter 380 that corresponds to thesmoothing filter 180, and another output of the ZCA module 370 providesone or more filter settings to the BPF 350. Corresponding components ofthe systems 100 and 300 are not necessarily identical and need notinclude the same functionality.

[0032] Operation

[0033] Referring to FIG. 4, a process 400 for propagating a signalthrough a vortex flow meter having a pre-filter and an amplitudedetector includes converting pressure to an electric signal (410), andoptionally amplifying the electric signal (420). The process 400includes determining if the amplitude of the electric signal is lessthan a low-flow threshold (430), and, if so, filtering the electricsignal with a pre-filter (440). The process 400 also includes filteringthe electric signal based on an estimated frequency (450), andoptionally converting the electric signal from analog to digital (460).Finally, the process 400 includes estimating a frequency of the electricsignal using a ZCA (470) and smoothing the frequency estimate (480).

[0034] Referring again to FIG. 1, a description of the propagation of asignal through the system 100 may be used to illustrate animplementation of the process 400. The pressure sensor 110 may becoupled, for example, to a bluff body that produces vortex shedding orto a pipe wall near the bluff body. The pressure sensor 110 senses thepressure variations of the vortex shedding and produces an electricsignal referred to as the vortex signal (410). The pre-amplifier 120amplifies the vortex signal (420) and provides the amplified vortexsignal to the pre-filter 130 and the amplitude detector 140. Theamplitude detector 140 determines whether the amplitude of the vortexsignal is less than a threshold amplitude (430). As explained earlier,the amplitude of the vortex signal is directly related to the flow rateof the vortex signal. Thus, the threshold amplitude corresponds to athreshold flow rate.

[0035] Before the amplitude detector 140 operates, the thresholdamplitude is set. The threshold amplitude is the maximum amplitude atwhich the pre-filter 130 will be enabled, and corresponds to the maximumflow rate at which the pre-filter 130 will be enabled. The thresholdamplitude may be chosen to be larger than some measure of the noise onthe vortex signal. One measure of the noise may be obtained by using aprobability distribution function of the noise to determine a noisevalue that is expected to be greater than ninety-nine percent of theencountered noise. Such a value is often referred to as a “three-sigma”point. The threshold amplitude may also be chosen such that thecorresponding flow rate is higher than the flow rate at which the ZCAempirically begins to fail. The amplitude detector 140 may use ahysteresis loop or other mechanism to inhibit the amplitude detector 140from oscillating rapidly between enabling and disabling the pre-filter130.

[0036] If the amplitude of the vortex signal is less than the thresholdamplitude (430), then the amplitude detector 140 enables the pre-filter130 (440). Conversely, if the amplitude detector 140 determines that theamplitude of the vortex signal is greater than the threshold amplitude,then the amplitude detector 140 disables the pre-filter 130. Whenenabled, the pre-filter 130 acts as a BPF and filters out high frequencynoise and direct current (“DC”) offset arising from the pre-amplifier120 (440). When disabled, the pre-filter 130 acts as an all-pass filterand has little or no effect on the vortex signal.

[0037] Before the pre-filter 130 is enabled, the pass band of thepre-filter 130 is set. One implementation sets the low end of the passband, f_(L,PF), where PF stands for pre-filter, to one-half of theminimum vortex frequency, and sets the high end of the pass band,f_(H,PF), to the vortex frequency corresponding to the thresholdamplitude. The minimum vortex frequency is the minimum frequency atwhich vortex shedding will reliably occur.

[0038] The pass band for the BPF 150 must also be set. Initially, thepass band is set so that the BPF 150 acts as an all-pass filter, havinglittle or no effect on the vortex signal. After the ZCA module 170 hasestimated the vortex frequency, the ZCA module 170 provides theestimated vortex frequency to the BPF 150 and the BPF 150 as a filtersetting establishes a pass band around the estimated vortex frequency.The pass band may be set in a variety of ways. One implementation setsthe low end of the pass band, f_(L,BPF), to one-half of the estimatedvortex frequency, and sets the high end of the pass band, f_(H,BPF), totwice the estimated vortex frequency. These settings provide for anarrower band at low frequencies than at high frequencies, whichcorresponds to a narrower band at low flow rates than at high flowrates.

[0039] After the pass band is established around the estimated vortexfrequency, the BPF 150 may filter the vortex signal using the pass band(450). The ADC 160 converts the filtered vortex signal from an analogsignal to a digital signal (460). The output of the ADC 160 is providedto the ZCA module 170 and may be binary or have a higher number ofdiscrete levels.

[0040] The ZCA module 170 may include functionality intended to rejectnoise. Such functionality may include, for example, using a hysteresisloop or other mechanism to inhibit the ZCA module 170 from recognizingzero crossings caused by noise. For example, a Schmitt Trigger or otherdevice with built-in hysteresis may be used. In one implementation, thenoise-rejecting mechanism uses a hysteresis value, h_(ZCA), to produce abinary output of either a higher value or a lower value. Thenoise-rejecting mechanism may produce a transition from the higher valueto the lower value if the input is below a negative value that has anabsolute value greater than h_(ZCA). Conversely, the noise-rejectingmechanism may produce a transition from the lower value to the highervalue if the input is greater than h_(ZCA). Each such transition mayindicate a zero crossing for the vortex signal. Other implementationsmay operate differently, such as, for example, by using differenthysteresis values for one or more of the two transitions, or bytransitioning upon different conditions.

[0041] The ZCA module 170 continues to estimate the vortex frequency(470) and provides filter settings to the BPF 150. As explained above,the filter settings provided may include an estimated vortex frequency.

[0042] The ZCA module 170 provides the estimated vortex frequency to thesmoothing filter 180. The smoothing filter 180 smoothes the estimates ofthe vortex frequency to produce a smoothed frequency estimate (480).

[0043] Referring again to FIG. 3, the propagation of a signal throughthe system 300 may be used to illustrate a variation of the process 400in which the analog-to-digital conversion occurs earlier than thatspecified in the process 400. The pressure sensor 310 converts pressureto an electric vortex signal (410) that is provided to the pre-amplifier320 which amplifies the electric vortex signal (420). The ADC 360converts the amplified electric vortex signal to a digital signal (460).The amplitude detector 340 determines if the amplitude of the digitalsignal is less than a low-flow threshold (430), and controls thepre-filter 330 to filter the digital signal if the amplitude is lessthan the low-flow threshold (440). The BPF 350 filters the digitalsignal based on an estimated frequency from the ZCA module 370 (450).The ZCA module 370 optionally performs a noise-rejecting function anduses a zero crossing algorithm to estimate the vortex frequency (470).Finally, the smoothing filter 380 smoothes the estimated vortexfrequency (480).

[0044] Referring to FIG. 5, a process 500 for determining if a vortexsignal has an amplitude that is less than a threshold amplitude includesdetermining the absolute value of the vortex signal (510), anddetermining the peak values of the absolute value vortex signal (520).The process 500 includes filtering the peak values (530) and comparingthe filtered peak values to the threshold amplitude (540).

[0045] Referring again to FIG. 2, a description of the propagation of asignal through the system 200 may be used to illustrate animplementation of the process 500. The absolute value converter 210determines the absolute value of the input vortex signal (510), and thepeak detector 220 determines the peaks of the converted vortex signal(520). The LPF 230 filters the peak values (530). Finally, thecomparator 240 compares the filtered peak values to the thresholdamplitude (540) to determine whether the threshold has been met.

[0046] The comparator 240 may use a hysteresis loop or other mechanismto inhibit rapid oscillations between enabling and disabling thepre-filter 130. In one such implementation, a hysteresis value, h_(C),is used and the comparator 240 only transitions to the state of enablingthe pre-filter 130 (from a state of having the pre-filter 130 disabled)if the detected amplitude falls below A-bar minus h_(C). Similarly, inthat implementation, the comparator 240 only transitions to the state ofdisabling the pre-filter 130 (from a state of having the pre-filter 130enabled) if the detected amplitude rises above A-bar plus h_(C). Otherimplementations may use different hysteresis values for one or more ofthe two transitions.

[0047]FIG. 6 shows simulation data indicating that variousimplementations of the system 300 may increase the ability of a vortexflow meter to lock on to a low-flow-rate vortex signal at start-up. Forexample, if the system 300 is started with a low flow rate, it may bedifficult for the ZCA module 370 (acting without the amplitude detector340 and the pre-filter 330) to lock on to the vortex signal. That is,the ZCA module 370 may be unable to detect the zero crossings that aredue to the vortex signal, as opposed to noise. Until a frequency isestimated, the BPF 350 may be acting as an all-pass filter and may notbe filtering any noise, and the ZCA module 370 may continue to have thesame level of noise. The amplitude detector 340, however, may be able tomore accurately detect that the flow rate is low, and may enable thepre-filter 330 to assist the ZCA module 370 in locking on to the vortexsignal.

[0048] As shown in FIG. 6, the input vortex signals of graphs 610-630each reflect start-up of a noisy system at a low flow rate. The flowrates decrease from 0.2 liters/second (“l/s”) to 0.1 l/s as shown ingraphs 640-660, while the noise increases as shown in graphs 610-630. Ineach case, however, the system produces accurate estimates of the flowrates, as indicated in the graphs 640-660.

[0049]FIGS. 7 and 8 show simulation and real data, respectively,indicating that various implementations of the system 300 may increasethe ability of a vortex flow meter to track a vortex signal from higherflow rates to lower flow rates where the SNR is lower. For example, theZCA module 370 (acting without the amplitude detector 340 and thepre-filter 330) may have difficulty detecting zero crossingsattributable to a vortex signal, as opposed to zero crossingsattributable to noise, as the flow rate decreases. The amplitudedetector 340, however, may be able to recognize that the flow rate isdecreasing and enable the pre-filter 330 to assist the ZCA module 370 totrack the vortex signal as the flow rate decreases.

[0050] Referring to the simulation data in FIG. 7, a graph 710 showsthat a particular conventional ZCA that tracks the actual flow rate downto 0.2 l/s does not track the flow rate down to 0.1 l/s. However, agraph 720 shows that coupling the same conventional ZCA to a pre-filter,such as the pre-filter 330, permits the vortex flow meter to track theactual flow rate down to 0.1 l/s.

[0051] Details of the simulation associated with FIG. 7 are nowdiscussed. The simulation used a test system substantially the same asthe system 300 to produce the trace labeled “With prefilter” in thegraph 720. The simulation produced the trace labeled “Conventionalalgorithm” in the graph 710 by omitting the amplitude detector and thepre-filter from the test system. Other implementations of the simulationassociated with FIG. 7 may use different systems and/or parameters.

[0052] A vortex flow meter is used that has the following threecharacteristics. First, the meter has a K-factor of 9.45171 inverseliters (“l⁻¹”). The K-factor is generally, constant for a given flowmeter, and the vortex frequency equals the flow rate multiplied by theK-factor. Second, the meter has an amplitude flow ratio (“AFR”) of0.356. The AFR is a constant for a given flow meter size and fluiddensity, and the amplitude of the vortex signal equals the square of theflow rate multiplied by the AFR. If the fluid is compressible and mayhave a changing density, temperature and pressure can be measured orestimated and used to determine the density, and the AFR can be adjustedaccordingly. Standard equations relating these variables may be used.Third, the meter is a two-inch meter having a minimum reliable vortexfrequency of one Hertz (“Hz”).

[0053] The flow rate is varied in the simulation between 0.1 l/s to 3.0l/s, as indicated in FIG. 7. Accordingly, the corresponding ranges foramplitude and frequency can be determined. Using the AFR, the amplitudeof the vortex signal varies between 3.56 millivolts (“mV”) and 3.20volts (“V”). Using the K-factor, the vortex-shedding frequency variesbetween 1 Hz and 30 Hz.

[0054] Plant noise is assumed to be white Gaussian with zero mean andvariance of 1*10E-5 for all values of the flow rate. Accordingly, thethree-sigma point, which is the ninety-nine percent confidence interval,is approximately 0.01.

[0055] A hysteresis value, h_(ZCA), for a ZCA module of the test systemis selected to be greater than the three-sigma point of the plant noise.A value of 0.012 V is used. Accordingly, the ZCA module is expected tofail for amplitude values that fall below 0.012 V. Using the AFR, theflow rate corresponding to an amplitude of 0.012 V is approximately 0.2l/s.

[0056] A pre-filter of the test system is configured so that it switcheson below a Reynolds number of 10,000. The Reynolds number (“Re”) isequal Go the fluid velocity (“vel”) times the diameter of the pipe line(“diam”) divided by the viscosity of the fluid (“visc”), orRe=vel*diam/visc.

[0057] Q-bar is related to Re by the following equation,Q-bar−(25*pi/4)(visc*S*Re), where S is the size of the flow meter ininches. For S=2 and visc=1*10E-6 (the viscosity of water), Q-bar isapproximately 0.4 l/s. Alternatively, Q-bar can be selected to be about0.4 l/s, and Re can be determined from Q-bar. In either case, thepre-filter will be switched on at a threshold flow rate of 0.4 l/s,which, using the AFR, corresponds to a threshold amplitude of 0.057 V.

[0058] A hysteresis value, h_(C), for an amplitude detector of the testsystem can also be selected as the three-sigma point of the plant noise.Thus, h_(C) can be set to 0.012 V. Other implementations may set one ormore of the hysteresis values to some fraction, such as, for example,one-third, of the expected vortex signal amplitude.

[0059] The low end of the pass band for the pre-filter is selected to beone-half of the minimum reliable vortex frequency, f_(L,PF), or 0.5 Hz.The high end of the pass band for the pre-filter, f_(H,PF), is selectedto be the frequency corresponding to the threshold flow rate, which,using the K-factor, is approximately 4 Hz. Other implementations mayselect the high end of the pass band to be somewhat higher so as toreduce the chance of filtering out the vortex signal if the vortexfrequency is equal to or near the threshold frequency.

[0060] When the pre-filter is switched on, the pre-filter removes someof the noise and will enable the ZCA module to track the vortexfrequency down to 0.1 l/s. A flow rate of 0.1 l/s corresponds to anamplitude of 0.00356 V, which is smaller than the hysteresis value,h_(ZCA), in the ZCA module. Accordingly, h_(ZCA) is modified to besmaller than 0.00356, and a value of 0.001 is selected. The ZCA modulemay therefore have a different hysteresis value when the pre-filter isenabled. The ZCA module may have multiple hysteresis values, or even acontinuously changing hysteresis value, depending, for example, on thesize and location of the pass band of a BPF of the test system.

[0061] A smoothing filter of the test system is assumed to have a timeconstant of one second.

[0062] As the graph 720 illustrates, the simulation tracks the flow ratedown to 0.1 l/s. This reflects an increase in the meter's turn-down,ratio by a factor of two. The turn-down ratio reflects the range of flowrates that a meter can measure. Assuming that the meter, without thepre-filter, could measure flow rates between 0.2 l/s and 10 l/s, themeter would have a turn-down ratio of fifty (10 divided by 0.2). Withthe pre-filter, the turn-down ratio of the meter is improved to onehundred.

[0063] The above data demonstrates that the cascaded filters of the testsystem, that is, the pre-filter and the BPF, need not have nested passbands for all frequencies at which both filters are enabled. Forexample, just below a flow rate of 0.4 l/s, the pre-filter of the testsystem has a fixed pass band of 0.5 Hz to 4 Hz and the BPF has a passband of approximately 2 Hz to 8 Hz. These pass bands overlap, but arenot nested. At a flow rate of 0.1 l/s, however, the BPF has a pass bandof approximately 0.75 Hz to 3 Hz, which is nested within the pass bandof the pre-filter.

[0064] In particular implementations, the pass band of a pre-filter maybe nested within the pass band of a BPF for particular frequencies atwhich both filters are enabled. For example, if the pass band of the BPFis designed to extend from one-fourth the estimated frequency to fourtimes the estimated frequency, then at a flow rate of 0.15 l/s, the BPFhas a pass band of approximately 0.375 Hz to 6 Hz. At that flow rate,and assuming that the pass band of the pre-filter is calculated asabove, then the pass band of the pre-filter is 0.5 Hz to 4 Hz, which isnested entirely within the pass band of the BPF. However, assuming atriggering flow Tate of 0.4 l/s, the pass band of the BPF isapproximately 1 Hz to 16 Hz and the pass band of the pre-filter (0.5 Hzto 4 Hz) overlaps this range (1 Hz to 16 Hz) but is not nested withinit.

[0065] Implementations may provide for different configurations of thepass bands of included filters, such as, for example, having one passband nested within another over an entire range for which the filtersare enabled. A filter's pass band may also be varied in other ways, suchas, for example, by increasing the pass band when the estimated vortexfrequency changes. By increasing a pass band in this manner, a filtermay be less likely to shift a narrow pass band onto a noise spur andfilter out the vortex frequency.

[0066] The real data in FIG. 8 provides results that are similar to theresults of the simulation data in FIG. 7. A graph 810 shows that aparticular conventional ZCA is unable to track the flow rate down past0.2 l/s. As with the simulation data in the graph 710, the real data inthe graph 810 shows that the conventional ZCA continues to output avalue of approximately 0.2 l/s even when the flow rate has dropped to0.1 l/s. This may be due, for example, to the behavior of a BPF, andillustrates one example of the unreliability of the conventional ZCA atlow flow rates. Analogous to the graph 720, a graph 820 shows thatcoupling the same conventional ZCA to a pre-filter, such as thepre-filter 330, permits the vortex flow meter to track the actual flowrate down to 0.1 l/s. The actual flow rate is taken to be the flow rateindicated by a reliable flow meter, labeled as “MM” in the legends ofthe graphs 810 and 820.

[0067]FIG. 9 shows real data indicating that various implementations ofthe system 300 may increase the ability of a vortex flow meter tocontinue to track a vortex signal at a low flow rate in the presence ofnoise. For example, even if the ZCA module 370 (acting without theamplitude detector 340 and the pre-filter 330) locks on to or tracks alow flow-rate vortex signal, large noise spurs may cause zero crossingsthat the ZCA module 370 improperly attributes to the vortex signal. Thepre-filter 330, however, which would presumably be enabled at the lowflow rate, may filter out the large noise spurs sufficiently to preventa zero crossing. Even if the ZCA module 370 (acting without theamplitude detector 340 and the pre-filter 330) were to recover and starttracking the signal again after improperly recognizing a zero crossingcaused by a large noise spur, the flow meter may produce inaccurate flowvalues during the time required for recovery.

[0068] A graph 910 shows that at a flow rate of 0.15 l/s, a particularconventional ZCA follows a large noise spur at approximately fiveseconds on the time axis. The ZCA may be said to be tracking the noisespur or to have lost lock. The graph 910 reveals that it takes the ZCAapproximately five seconds, until approximately ten seconds on the timeaxis, to recover from the noise spur and estimate an accurate flow rate.Additional spurs at approximately sixteen seconds and thirty-threeseconds also cause the ZCA to mis-estimate the vortex frequency. A graph920 shows that coupling the same conventional ZCA to a pre-filter, suchas the pre-filter 330, permits the vortex flow meter to track the actualflow rate despite the presence of the noise spurs. As in FIG. 8, theactual flow rate is taken to be the flow rate indicated by a reliableflow meter, labeled as “MM” in the legends of the graphs 910 and 920.

[0069] Details of the system associated with the real data of FIGS. 8and 9 are now discussed. A test system substantially the same as thesystem 300 was used to produce the traces labeled “With PF” in thegraphs 820 and 920. The traces labeled “Transmitter” in the graphs 810and 910 were produced with a conventional vortex flow meter that doesnot include an amplitude detector and a pre-filter. In both the testsystem and the conventional vortex flow meter, substantially all of thefunctionality occurring after the pre-amplifier stage is performed by adigital signal processor (“DSP”) chip or chip set. Other systems mayuses one or more discrete components. The conventional vortex flow metermay allow the analog data from a pressure sensor or a pre-amplifier tobe tapped. Using such a tap as an input, the test system may produce thereal data in the “With PF” traces of the graphs 820 and 920.

[0070] Noise immunity may be further enhanced because, after the flowrate is determined to be low, the systems 100-300 may assume that theflow rate will not change quickly, which allows the systems to use theLPF 230 or a digital equivalent which may filter out large noise spursand/or other higher-frequency noise. Implementations may also use ahysteresis loop to filter out noise in the determination of theamplitude. These features may inhibit the amplitude detectors 140 and340 from improperly disabling the pre-filters 130 and 330, and maythereby increase the robustness of the amplitude detectors 140 and 340.The robustness of the amplitude detectors 140 and 340 may be furtherenhanced by the fact that the amplitude detectors 140 and 340 may beconcerned only with detecting a range of low flow rates, and, therefore,may not be concerned with amplitude changes that do not put theamplitude out of that range. Conversely, the ZCA modules 170 and 370 maybe concerned about every change in frequency.

[0071] Additional Examples

[0072] Referring to FIG. 10, a system 1000 may be used as a vortex flowmeter to measure flow rate using the vortex shedding principle. Thesystem 1000 includes a pressure sensor 1010 that corresponds to thepressure sensor 110 and provides an output to a pre-amplifier 1020 thatcorresponds to the pre-amplifier 120. The output of the pre-amplifier1020 is provided to an amplitude detector 1040 that corresponds to theamplitude detector 140. The output of the pre-amplifier 102 is alsoprovided to a combined filter 1090 that is controlled, at least in part,by the output of the amplitude detector 1040. The combined filter 1090filters an input signal and provides an output to an ADC 1060 thatcorresponds to the ADC 160. The ADC 1060 provides an output to a ZCAmodule 1070 that corresponds to the ZCA module 170. One output of theZCA module 1070 provides a filter setting to the combined filter 1090,and another output of the ZCA module 1070 is provided to a smoothingfilter 1080 that corresponds to the smoothing filter 180. Thecorresponding components of the systems 100 and 1000 are not necessarilyidentical and need not include the same functionality.

[0073] The combined filter 1090 may perform the same, or different,filtering functions from those performed by the pre-filter 130 and theBPF 150. In one implementation, the combined filter 1090 performs thesame functions, acting as a two-mode filter. In the first mode, only thefiltering of the BPF 150 is performed by the combined filter 1090. Inthe second mode, the filtering of both the BPF 150 and the pre-filter130 is performed by the combined filter 1090. Such a combined filter1090 may consist, for example, of a filter module having only a singlefilter architecture or a filter module including two separate filters.Other implementations of the combined filter 1090 only perform thefiltering of the pre-filter 130, ignoring the feedback provided by theZCA module 1070. Still other implementations of the combined filter 1090perform the filtering of a single BPF with a single pass band beingcontrolled by both the amplitude detector 1040 and the ZCA module 1070.

[0074] Referring to FIG. 11, a system 1100 illustrates a longitudinalcross-section of a pipe 1110 containing a bluffbody 1120 that inducesvortex shedding in a fluid flowing in the pipe 1110. An arrow 1130indicates the direction of flow.

[0075] Additional Variations

[0076] Referring again to FIG. 1, the pressure sensor 110 may include,for example, a differential pressure sensor or an absolute pressuresensor, and may include materials such as, for example, a piezoelectricmaterial. Thus, one example of a pressure sensor is a piezoelectricdifferential pressure sensor. The pre-amplifier 120 may include, forexample, an electronic component that amplifies an electric signal, or acomponent designed to work with signals such as, for example,electromagnetic and optical signals. The pre-filter 130 may include, forexample, a BPF and a LPF. The BPF 150, as well as other filters, mayinclude, for example, an electronic filter or filtering module in whichthe range of frequencies that are passed is fixed or can be set by auser. The BPF 150 may include filters designed for electric,electromagnetic, optical, or other signals. The ADC 160 may include, forexample, a converter designed to work with electric, electromagnetic,optical, or other signals. The noise-rejecting mechanism described asbeing part of the ZCA module 170 may also, or alternatively, beincorporated in the ADC 160. One example is a Schmitt Trigger. The ZCAmodule 170 may include, for example, a device capable of detectingtransitions in a signal, or a comparator capable of comparing a signalto a known value such as zero. The ZCA module 170 may also include aprocessor, an arithmetic unit, a switch, a relay, or another devicecapable of determining the time between zero crossings and thecorresponding frequency. Further, the ZCA module 170 may include one ormore additional components, such as a comparator or a logic device, toimplement a hysteresis loop. The same, or different, component(s) may beused to perform the zero crossing detection and the frequencyestimation. The smoothing filter 180 may include, for example, a LPF, aBPF, or another filter or filtering module capable of smoothing thefrequency estimates.

[0077] Various ones of the components 110-180 in the system 100 may beomitted or moved. For example, the ADC 160 may be moved so that it isafter the ZCA module 170, the smoothing filter 180, or the pre-amplifier120 as in FIG. 3, or may be omitted altogether. Further,pre-amplification may not be needed in all implementations or may beneeded at different, and potentially multiple, points in animplementation. As discussed earlier, the pre-filter 130 and the BPF 150may be combined in various ways. Additional filters may also be desiredin various locations of an implementation. Further, another component,such as, for example, a resistor, a capacitor, an isolator, or anoperational amplifier, may be desired in one or more of variouslocations of an implementation.

[0078] Referring again to FIG. 2, the absolute value converter 210 mayinclude, for example, a rectifier or another device capable of providingthe absolute value of a signal. A DC component may be subtracted fromthe input to the absolute value detector 210 before the absolute valueis determined. The peak detector 220 may include, for example, acomparator or a slope detector. The peak detector 220 may detect arelative minimum and maximum without regard for whether the value ispositive or negative and may also compute the difference between asuccessive relative-maximum (peak) and relative-minimum (valley) if a DCcomponent is present. The comparator 240, and other comparators, mayinclude, for example, an electronic comparator, a subtracter, aprocessor, a relay, a switch, or another device capable of comparing twovalues. Further, the comparator 240 may include one or more additionalcomponents, such as, for example, a comparator and a logic device orcircuit, to implement a hysteresis loop. The output of the peak detector220, the LPF 230, or the comparator 240 may be referred to equivalentlyas, for example, a detected amplitude or an estimated amplitude.

[0079] Various ones of the components 210-240 in the system 200 may beomitted or moved. For example, the absolute value converter 210 may beomitted or moved to the location after the peak detector 220. The LPF230 may be placed before the peak detector 220 or may be omittedaltogether. The function of the comparator 240 may be integrated intoanother device, such as, for example, the peak detector 220 or thepre-filter 130. An additional component, such as, for example, a filter,a resistor, a capacitor, an isolator, or an operational, amplifier, maybe desired in one or more of various locations of an implementation.

[0080] The amplitude detector 140 may operate in a different manner thanthat previously described. For example, the input signal may be comparedto the threshold amplitude without taking the absolute value, detectingthe amplitude, and/or filtering. In such an implementation, a relay, aswitch, or another comparator may be used, with or without hysteresis,to directly turn the pre-filter 130 on or off. As another example, a“square-law” method may be used that includes squaring the signal andthen filtering the squared signal. These methods may be used in eitheranalog or digital implementations.

[0081] Referring again to FIG. 3, the functionality of the ADC 360, theamplitude detector 340, the pre-filter 330, the BPF 350, the ZCA module370, and the smoothing filter 380 may be performed using, for example,discrete components, programmable logic devices, DSPs, or otherprocessors. One or more additional components may also be added, asindicated above for the systems 100 and 200. The system 300 may alsoinclude additional functionality, such as, for example, a user interfaceto a DSP or other device allowing modification of various parameters foroperational, testing, or other purposes.

[0082] Various operations in the processes 400 and 500 may be performedin different orders or eliminated altogether. Some examples are providedby examining the variations discussed for the systems 100-300.

[0083] The simulation and real data provided in FIGS. 6-9 is notintended to be limiting. The data illustrates certain features ofvarious implementations. The features may be illustrated by otherimplementations and need not be illustrated by all, implementations.

[0084] The system 1000 may be modified in a variety of ways, such as,for example, those described for the systems 100-300.

[0085] A number of implementations have been described. Nevertheless, itwill be understood that various modifications may be made. For example,the components of the described systems may generally be implemented ineither analog or digital technology, or a combination of the two.Additionally, the components of the described systems may generally beinterchanged, as may the operations of the described methods.Accordingly, other implementations are within the scope of the followingclaims.

1-20. (Cancelled).
 21. A method of determining a flow rate of a fluid,the method comprising: determining that a flow has low flow rate bydetecting an amplitude of a vortex signal arising from the flow;filtering the vortex signal to reduce a high-frequency component, basedon the determination that the flow has low flow rate; and determining aflow rate of the flow using a zero-crossing algorithm on the filteredvortex signal.
 22. The method of claim 21 wherein detecting an amplitudecomprises: detecting peaks in the amplitude of the vortex signal; andfiltering the detected peaks in the amplitude of the vortex signal toremove a high-frequency component.
 23. The method of claim 21 whereindetecting the amplitude of the vortex signal comprises: detecting peaksof the vortex signal; and filtering the detected peaks to reducehigh-frequency components.
 24. The method of claim 21 wherein the flowis in a vortex flowmeter and determining that the flow has low flow ratecomprises determining that the flow in the vortex flowmeter has low flowrate.
 25. A method comprising: determining that a flow has low flow ratebased on an amplitude of a vortex signal arising from the flow;filtering the vortex signal to reduce a high-frequency component, basedon the determination that the flow has low flow rate; and estimating afrequency of the vortex signal.
 26. The method of claim 25 wherein thefrequency is related to a flow rate of the flow.
 27. The method of claim25 further comprising determining a flow rate of the flow using theestimated frequency.
 28. The method of claim 25 wherein estimating thefrequency comprises using a zero-crossing algorithm on the filteredvortex signal.
 29. The method of claim 25 wherein determining that theflow has low flow rate based on the amplitude of the vortex signalcomprises: comparing the amplitude of the vortex signal to a threshold;and classifying the flow as having low flow rate if the amplitude isless than the threshold.
 30. The method of claim 29 wherein filteringthe vortex signal comprises filtering the vortex signal, using a passband that excludes the high-frequency component, if the amplitude isless than the threshold.
 31. A method of processing a vortex signal, themethod comprising: comparing an amplitude of a vortex signal to athreshold amplitude; producing an indication of whether the amplitude ofthe vortex signal is less than the threshold amplitude; filtering thevortex signal using a first pass band only if the amplitude of thevortex signal is less than the threshold amplitude; filtering the vortexsignal using a second pass band if the first pass band is not used; andestimating a vortex frequency of the filtered vortex signal.
 32. Themethod of claim 31 wherein estimating the vortex frequency comprisesdetecting zero crossings of the filtered vortex signal.
 33. The methodof claim 31 wherein the vortex signal arises from a flow in a vortexflow meter and the method further comprises determining a flow rate ofthe flow based on the estimated vortex frequency.
 34. The method ofclaim 31 wherein the threshold amplitude reflects a low flow rate, suchthat the vortex signal is filtered using the first pass band only if theflow rate is low.
 35. The method of claim 34 wherein the first pass banddoes not vary with the amplitude of the vortex signal.
 36. The method ofclaim 31 wherein the threshold amplitude is adjusted by a hysteresisvalue.
 37. The method of claim 31 wherein the second pass band is anall-pass band, and filtering the vortex signal using the second passband if the first pass band is not used comprises filtering the vortexsignal using the all-pass band if the first pass band is not used.
 38. Adigital signal processor capable of performing at least the followingfunctions: determining that a flow has low flow rate based on anamplitude of a vortex signal arising from the flow; filtering the vortexsignal to reduce a high-frequency component, based on the determinationthat the flow has low flow rate; and estimating a frequency of thevortex signal.
 39. A digital signal processor capable of performing atleast the following functions: determining that a flow has low flow rateby detecting an amplitude of a vortex signal arising from the flow;filtering the vortex signal to reduce a high-frequency component, basedon the determination that the flow has low flow rate; and determining aflow rate of the flow using a zero-crossing algorithm on the filteredvortex signal.
 40. A digital signal processor capable of performing atleast the following functions: comparing an amplitude of a vortex signalto a threshold amplitude; producing an indication of whether theamplitude of the vortex signal is less than the threshold amplitude;filtering the vortex signal using a first pass band only if theamplitude of the vortex signal is less than the threshold amplitude;filtering the vortex signal using a second pass band if the first passband is not used; and estimating a vortex frequency of the filteredvortex signal.